U.S. patent number 6,967,698 [Application Number 10/252,028] was granted by the patent office on 2005-11-22 for plane light source apparatus.
This patent grant is currently assigned to Omron Corporation. Invention is credited to Masayuki Shinohara, Junichi Takagi, Yasuhiro Tanoue, Junichiro Tsuji.
United States Patent |
6,967,698 |
Tanoue , et al. |
November 22, 2005 |
Plane light source apparatus
Abstract
To make an efficiency of utilizing light with high efficiency in
a plane light source apparatus used along with a liquid crystal
display panel of a reflection type as a backlight, light f3 leaked
from a rear face of a light guide plate 25 in a skewed direction,
is incident on inside of a prism sheet 27 and reflected by a
reflecting face 32. Light f3 reflected by the reflecting face 32,
is reflected in a direction orthogonal to the prism sheet 27,
passes the light guide plate 25 and is emitted from a light
emitting face 30 of the light guide plate 25 in a direction
orthogonally thereto.
Inventors: |
Tanoue; Yasuhiro (Kusatsu,
JP), Shinohara; Masayuki (Nagaokakyo, JP),
Takagi; Junichi (Kyoto, JP), Tsuji; Junichiro
(Kusatsu, JP) |
Assignee: |
Omron Corporation (Kyoto,
JP)
|
Family
ID: |
19112441 |
Appl.
No.: |
10/252,028 |
Filed: |
September 20, 2002 |
Foreign Application Priority Data
|
|
|
|
|
Sep 21, 2001 [JP] |
|
|
2001-290044 |
|
Current U.S.
Class: |
349/62;
349/64 |
Current CPC
Class: |
G02B
6/0028 (20130101); G02B 6/0031 (20130101); G02B
6/0036 (20130101); G02B 6/0038 (20130101); G02B
6/0046 (20130101); G02B 6/0053 (20130101); G02B
6/0055 (20130101); G02B 6/0061 (20130101); G02B
6/0071 (20130101) |
Current International
Class: |
F21V
8/00 (20060101); G02F 001/1335 () |
Field of
Search: |
;349/61-65 ;362/31 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Nguyen; Dung T.
Attorney, Agent or Firm: Osha Liang LLP
Claims
What is claimed is:
1. A plane light source apparatus comprising: a light source; and a
light guide plate for confining light from the light source and
emitting the light from a surface thereof; wherein an optical sheet
for reflecting light emitted from a rear face of the light guide
plate in a skewed direction and making the light incident on the
rear face of the light guide plate again orthogonally to the rear
face is arranged opposite to the rear face of the light guide
plate, wherein the optical sheet is formed with a recessed and
projected pattern on a face thereof on a side opposed to a face
thereof opposed to the light guide plate, and the face formed with
the recessed and projected pattern defines a light reflecting face,
wherein the rear face of the light guide plate is discretely formed
with a diffusing pattern for reflecting light in the light guide
plate, and the light reflected by the diffusing pattern is emitted
from the surface of the light guide plate in a direction
substantially orthogonal to the surface of the light guide plate,
and wherein a directing angle of the light reflected by the optical
sheet and then emitted from the light guide plate is narrower than
a directing angle of the light reflected by the diffusing pattern
and then emitted from the light guide plate.
2. The plane light source apparatus according to claim 1, wherein
the light reflecting face is produced by forming a metal thin film
on the face formed with the recessed and projected pattern.
3. The plane light source apparatus according to claim 1, wherein
the light reflected by the optical sheet and made incident on the
rear face of the light guide plate again, is emitted from a surface
of the light guide plate in a direction substantially orthogonal to
the surface of the light guide plate.
4. The plane light source apparatus according to claim 1, wherein
the light source is a light source extended along an end face of
the light guide plate and the recessed and projected pattern formed
at the optical sheet is a pattern in parallel with the light
source.
5. The plane light source apparatus according to claim 1, wherein
the light source is a point light source and the recessed and
projected patterns formed at the optical sheet and diffusing
patterns formed at the light guide plate are respectively arranged
in a shape of concentric circles centering on the light source.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a plane light source apparatus,
particularly to a plane light source apparatus used as a backlight
in an image display apparatus of a liquid crystal display apparatus
or the like.
2. Description of the Related Art
FIG. 1 is a disassembled perspective view showing a structure of a
plane light source apparatus 1 of a related art and FIG. 2 is a
sectional view thereof. The plane light source apparatus 1 is
constituted by a light emitting portion 2, a light guide plate 3, a
reflecting plate 4 and a prism sheet 5. The light emitting portion
2 is mounted with a plurality of pieces of so-to-speak point light
sources 7 of light emitting diodes or the like on a circuit board
6, and arranged opposedly along an end face (light incident end
face 8) of the light guide plate 3. The light guide plate 3 is
molded by a transparent resin having a large refractive index such
as polycarbonate resin or methacrylic resin, and at a lower face
(face on a side opposed to a light emitting face 9) of the light
guide plate 3, there is formed a diffusing pattern 10 by recessed
and projected forming, dot printing of diffusing and reflecting ink
or the like. The reflecting plate 4 is formed by, for example, a
white resin sheet having high reflectance and both end portions of
the reflecting plate 4 are pasted to the lower face of the light
guide plate 3 by a double coated tape 11. The prism sheet 5 is
arranged with prism patterns 12 having a section in a triangular
shape in parallel with each other and placed above the light
emitting face 9 of the light guide plate 3.
Further, according to the plane light source apparatus 1, as shown
by FIG. 2, light f1 emitted from the light emitting portion 2, is
introduced from the light incident end face 8 of the light guide
plate 3 to an inner portion of the light guide plate 3. The light
f1 confined at inside of the light guide plate 3, progresses at the
inner portion of the light guide plate 3 while repeating total
reflection by an upper face and a lower face of the guide plate 3.
At this occasion, when the light f1 incident on the diffusing
pattern 10 and diffused and reflected by the diffusing pattern 10,
is incident on the light emitting face 9 of the light guide plate 3
by an angle of incidence smaller than a critical angle of total
reflection, as shown by FIG. 2, the light f1 is emitted from the
light emitting face 9 to outside of the light guide plate 3. Light
emitted skewedly from the light emitting face 9 of the light guide
plate 3, passes the prism sheet 5 and is bent to a direction
orthogonal to the plane light source apparatus 1 by the prism sheet
5. Further, the light f1 emitted from the lower face of the light
guide plate 3 by passing a portion of the lower face of the light
guide plate 3 where the diffusing pattern 10 is not present,
returns to inside of the light guide plate 3 again by being
reflected by the reflecting plate 4 to thereby prevent loss of an
amount of light from the lower face of the light guide plate 3.
FIG. 3 is an outline view for explaining operation of the prism
sheet 5 in the plane light source apparatus 1. In light diffused
and reflected by the diffusing pattern 10, only light incident on
the light emitting face 9 by an angle of incidence smaller than the
critical angle of total reflection, is emitted from the light
emitting face 9 of the light guide plate 3, further, since a
diffusing degree of the diffusing pattern 10 is not so much large,
the light f1 is emitted from the light emitting face 9 with a
comparatively narrow directing angle in a direction inclined from a
direction orthogonal to the light emitting face 9. Now, as shown by
FIG. 3, considering a case in which the prism sheet 5 is opposed to
the upper face of the light guide plate 3 by directing the prism
pattern 12 to a side opposed to the light guide plate 3, light
skewedly incident on the lower face of the prism sheet 5, is
refracted in a direction substantially orthogonal to the prism
sheet 5 by refracting operation of the prism pattern 12. As a
result, light passing the prism sheet 5, is emitted in the
direction substantially orthogonal to the prism sheet 5. FIG. 5 is
a graph showing a relationship between an angle of emittance from
the prism sheet 5 and a brightness (rate) by constituting a
reference by the direction orthogonal to the prism sheet 5, and a
bold line curve C1 in FIG. 5 shows a relationship between the angle
of emittance and the brightness when the prism pattern 12 is
arranged to direct to the side opposed to the light guide plate 3
as shown by FIG. 3.
Further, as shown by FIG. 4, even when the prism sheet 5 is made to
be opposed to the upper face of the light guide plate 3 by
directing the prism pattern 12 having an apex angle of 60.degree.
to the side of the light guide plate 3, light skewedly incident on
the prism pattern 12 on the lower face of the prism sheet 5, is
refracted in the direction substantially orthogonal to the prism
sheet 5 by refracting operation of the prism pattern 12. A
relationship between the angle of emittance and the brightness in
the case of arranging the prism pattern 12 to direct to the side of
the light guide plate 3 in this way, is as shown by a broken line
curve C2 of FIG. 5.
Next, FIG. 6 is an outline sectional view showing a plane light
source apparatus 13 having other structure. According to the plane
light source apparatus 13, light f2 emitted from a light emitting
portion and incident on inside of a light guide plate 14 is
propagated in the light guide plate 14 while repeating total
reflection between an upper face and a lower face of the light
guide plate 14. The lower face of the light guide plate 14 is
formed with a reflecting pattern 15 in a shape of a triangular
prism, and as in light f2 shown in FIG. 6, the light f2 reflected
(total reflection or mirror reflection) twice by the lower face of
the light guide plate 14, is emitted orthogonally from the upper
face (light emitting face 16) of the light guide plate 14.
When a plane light source apparatus is used as a backlight of a
liquid crystal display panel, a viewing angle easy to see an image
of particularly, a liquid crystal display panel is said to be about
30.degree. (.+-.15.degree.). However, according to the plane light
source apparatus as shown by FIGS. 1 through 3 and the plane light
source apparatus as shown by FIG. 4, a directing property of light
emitted from the prism pattern 12 is wide as in the bold line curve
C1 or the broken line curve C2 shown in FIG. 5 and light is emitted
in other than the direction orthogonal to the prism pattern 12 and
loss of light is considerable.
For example, according to the plane light source apparatus as shown
by FIG. 1 through FIG. 3, as shown by FIG. 7, in order to
constitute a spreading angle of light passing through the prism
sheet 5 by 30.degree., it is necessary that a spreading angle of
light emitted from the light guide plate 3 is set to about
20.degree.. According to the plane light source apparatus disposed
with prism sheet 5 as shown in FIG. 4, in order to constitute a
spreading angle of light passing through the prism sheet 5 by
30.degree., it is necessary that also a spreading angle of light
emitted from the light guide plate 3 is set to about 30.degree..
However, actually, according to the plane light source apparatus,
the directing property is as wide as shown by FIG. 5 and
particularly, light emitted to outside of the viewing angle easy to
see, constitutes the loss.
Further, according to a liquid crystal display apparatus using the
plane light source apparatus placing the prism sheet 5 above the
upper face of the light guide plate 3 as shown by FIG. 3 or FIG. 4
and overlapping a liquid crystal display panel thereabove, the
prism sheet 5 is disposed right under the liquid crystal display
panel and therefore, there is brought about a drawback that a
defect having a size of about 0.1 mm of the prism sheet 5 is seen.
Further, since the prism sheet 5 is disposed below the liquid
crystal display panel, there is a high concern of producing Moire
fringes between pixels of the liquid crystal display panel and the
prism pattern 12 of the prism sheet 5.
Further, according to the plane light source apparatus 13 having
the structure as shown by FIG. 6, as shown by FIG. 9, light f2
leaked from the lower face of the light guide plate 14 can be
returned into the light guide plate 14 again by reflecting the
light by a reflecting plate 17 laid at the lower face of the light
guide plate 14. However, when the light f2 which has returned into
the light guide plate 14, passes the light guide plate 14, the
light f2 is emitted in a skewed direction from the upper face of
the light guide plate 14 and is not emitted in the direction
orthogonal to the light guide plate 14 to thereby constitute the
loss.
Therefore, according to the plane light source apparatus of the
related art, an efficiency of utilizing light emitted from the
light emitting portion is not sufficient and a plane light source
apparatus having a higher efficiency is requested.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a plane optical source
apparatus having a higher efficiency.
According to an aspect of the invention, there is provided a plane
light source apparatus comprising a light source, and a light guide
plate for confining light from the light source and emitting the
light from a surface thereof, wherein an optical sheet for
reflecting light emitted from a rear face of the light guide plate
in a skewed direction and making the light incident on the rear
face of the light guide plate again orthogonally to the rear face
is arranged opposedly to the rear face of the light guide plate. In
this case, the skewed direction is a direction inclined from a
direction orthogonal to the rear face of the light guide plate.
According to the plane light source apparatus of the invention, the
optical sheet for reflecting light emitted from the rear face of
the light guide plate in the skewed direction and making the light
incident on the rear face of the light guide plate orthogonally
thereto, is arranged opposedly to the rear face of the light guide
plate and therefore, light leaked from the rear face of the light
guide plate can be reflected into a viewing angle, light which has
been reflected outside of the viewing angle in the related art can
be condensed into the viewing angle and an efficiency of utilizing
light can be increased.
Further, according to the plane light source apparatus, when the
plane light source apparatus is arranged on a rear side of an image
display panel, the optical sheet is disposed at a position remotest
from the image display panel and therefore, even when there is a
defect in the optical sheet, the defect is made to be difficult to
see and Moire fringes are made to be difficult to be brought about
between the image display panel and the optical sheet.
An embodiment according to the plane light source apparatus of the
invention, is featured in that the optical sheet is formed with a
recessed and projected pattern on a face thereof on a side opposed
to a face thereof opposed to the light guide plate, and the face
formed with the recessed and projected pattern defines a light
reflecting face. According to the embodiment, light is reflected by
the reflecting face comprising the recessed and projected pattern
and therefore, a direction of reflecting light can be controlled by
a shape or an angle of inclination of the recessed and projected
pattern.
Particularly, the light reflecting face may be produced by forming
a metal thin film on the face formed with the recess and projected
pattern. According to the embodiment, since light incident on the
optical sheet can be reflected by the light reflecting face
comprising the metal thin plate and returned to the side of the
light guide plate and therefore, light can be reflected more firmly
than reflecting light by utilizing total reflection. Therefore,
there is not leakage of light from the rear face of the optical
sheet and the light utilizing efficiency can further be
promoted.
Other embodiment according to the plane light source apparatus of
the invention is featured in that the light reflected by the
optical sheet and made incident on the rear face of the light guide
plate again, is emitted from a surface of the light guide plate in
a direction substantially orthogonal to the surface of the light
guide plate. Light made to be incident on the rear face of the
light guide plate again substantially orthogonally thereto by the
optical sheet, is emitted from the surface of the light guide plate
substantially in the vertical direction and therefore, the
efficiency of utilizing light can extremely be promoted
thereby.
Still other embodiment according to the plane light source
apparatus of the invention is featured in that the rear face of the
light guide plate is discretely formed with a diffusing pattern for
reflecting light in the light guide plate, and the light reflected
by the diffusing pattern is emitted from the surface of the light
guide plate in a direction substantially orthogonal to the surface
of the light guide plate. According to the embodiment, also light
reflected in the light guide plate is reflected by the diffusing
pattern and emitted from the surface of the light guide plate
substantially in the vertical direction and therefore, light can be
condensed in a direction easy to see an image and the efficiency of
utilizing light can further be promoted.
Still other embodiment according to the plane light source
apparatus of the invention, is featured in that a directivity of
reflected light reflected by the optical sheet becomes narrower
than a directivity of incident light incident on the optical sheet.
According to the embodiment, the directivity of light can be
narrowed by reflecting light leaked from the light guide plate by
the optical sheet and therefore, the efficiency of utilizing light
can further be promoted.
Still other embodiment according to the plane light source
apparatus of the invention, is featured in that a directivity of
light propagating in the light guide plate is narrowed in one
direction when viewed from a direction orthogonal to the surface of
the light guide plate. According to the embodiment, the directivity
of light propagating in the light guide plate is narrowed in one
direction and therefore, also a directivity of light leaked from
the rear face of the light guide plate is narrowed in the one
direction and control of reflected light by the optical sheet is
facilitated.
According to still other embodiment of the plane light source
apparatus of the invention, the recessed and projected patterns
formed at the optical sheet and the diffusing patterns formed at
the light guide plate, may respectively be arranged in a shape of
concentric circles centering on the light source (for example,
point light source). Further, according to still other embodiment
of the plane light source apparatus of the invention, it is
preferable that in the case of a light source (so-to-speak line
light source) extended along an end face of the light guide plate,
the recessed and projected pattern formed at the optical sheet
becomes a pattern in parallel with the light source.
Further, constituent elements of the invention explained above can
be combined as arbitrarily as possible.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a disassembled perspective view showing a structure of a
plane light source apparatus of a related art;
FIG. 2 is a sectional view of the plane light source apparatus of
the same;
FIG. 3 is an outline view for explaining operation of a prism sheet
in the plane light source apparatus of the same;
FIG. 4 is an outline view for explaining operation of a prism sheet
in a plane light source apparatus having a different direction of a
prism pattern;
FIG. 5 is a graph showing a relationship between an angle of
emittance from a prism sheet and a brightness (rate) constituting a
reference by a direction orthogonal to the prism sheet, a bold line
carbon C1 shows a relationship between the angle of emittance and
the brightness when a prism pattern is arranged to direct to a side
opposed to a light guide plate, and a broken line curve C2 shows a
relationship between the angle of emittance and the brightness when
the prism pattern is arranged to direct to a side of the light
guide plate;
FIG. 6 is an outline sectional view showing a plane light source
apparatus having other structure;
FIG. 7 is a view showing a relationship between a viewing angle of
light incident on a prism sheet and a viewing angle of light
emitted from the prism sheet when a prism pattern is directed to a
side opposed to a light guide plate;
FIG. 8 is a view showing a relationship between a viewing angle of
light incident on a prism sheet and a viewing angle of light
emitted from the prism sheet when a prism pattern is directed to a
side of a light guide plate;
FIG. 9 is a view for explaining a behavior of light leaked from a
lower face of a light guide plate in the plane light source
apparatus shown in FIG. 6;
FIG. 10 is a disassembled perspective view showing a structure of a
liquid crystal display apparatus according to an embodiment of the
invention;
FIG. 11 is a sectional view of the liquid crystal display apparatus
of the same;
FIG. 12 is a view showing a diffusing pattern formed at a lower
face of a light guide plate;
FIGS. 13(a) and 13(b) are views for explaining a behavior of light
impinging on the diffusing pattern in the light guide plate;
FIG. 14(a) is a view showing a behavior of progressing of light at
the diffusing pattern of the light guide plate and inside of the
light guide plate, and FIG. 14(b) is a view showing a directivity
of light emitted from a light emitting face of the light guide
plate;
FIGS. 15(a) and 15(b) are a partially broken enlarged sectional
view showing an example of a resin-molded portion of a prism sheet
and a rear view thereof;
FIG. 16 is a view showing a behavior of light leaked from a rear
face of a light guide plate reflected by a prism sheet and incident
on inside of the light guide plate again;
FIG. 17 is a diagram showing a directing property of light
reflected by a diffusing pattern of a light guide plate and emitted
from an upper face of the light guide plate;
FIG. 18 is a diagram showing a directing property of light emitted
from a rear face of the light guide plate;
FIG. 19 is a view showing directing angles of light emitted from a
lower face of a light guide plate and incident on a prism sheet,
light at inside of the prism sheet and light emitted from an upper
face of the prism sheet;
FIG. 20 is a diagram showing a relationship between a pattern
density of a diffusing pattern provided at a rear face of a light
guide plate and an amount of light emitted from the light guide
plate (emitting light amount) as well as an inverse number of a
diffusing degree;
FIG. 21 is a disassembled perspective view showing a structure of a
liquid crystal display apparatus according to other embodiment of
the invention;
FIG. 22 is a perspective view showing a liquid crystal display
apparatus using a light emitting portion converting light of a
point light source to light of a line light source;
FIG. 23 is a outline side view showing a liquid crystal display
apparatus according to still other embodiment of the invention;
and
FIG. 24 is an outline side view showing a liquid crystal display
apparatus arranging a prism sheet to incline skewedly.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(First Embodiment)
FIG. 10 is a disassembled perspective view showing a structure of a
liquid crystal display apparatus 21 according to an embodiment of
the invention and FIG. 11 is a sectional view thereof. The liquid
crystal display apparatus 21 comprises a liquid crystal display
panel 22 of a transmission type, a diffusing plate 23 and a plane
light source apparatus (backlight) 24, and the plane light source
apparatus 24 is constituted by a light guide plate 25, a light
emitting portion 26 and a prism sheet 27. The light emitting
portion 26 is constituted by a so-to-speak point light source such
as a light emitting diode (LED) or the like. The point light source
needs not to be a single piece of a light emitting diode or the
like but a plurality of pieces of light emitting diodes or the like
may locally be present in areas smaller than a width of the light
guide plate 25. Further, the light emitting portion 26 may be
arranged to be opposed to a central portion of an end face (light
incident end face 28) of the light guide plate 25 or may be
embedded at an end portion of the light guide plate 25.
The light guide plate 25 is molded by a transparent resin material
having a large refractive index of polycarbonate resin or
methacrylic rein, and a lower face of the light guide plate 25 is
formed with a number of diffusing patterns 29. As shown by FIG. 12,
the diffusing patterns 29 are arranged in a shape of concentric
circles centering on the light emitting portion 26. The diffusing
pattern 29 is formed by a recessed portion having a section in a
triangular shape or a section substantially in a semicircular shape
(semicylindrical shape) formed at the lower face of the light guide
plate 25, and the respective diffusing pattern 29 is extended in a
direction orthogonal to a direction of line connecting the light
emitting portion 26 and a position of arranging the diffusing
pattern 29.
Further, light emitted from the light emitting portion 26 enters
the light guide plate 25 from the light incident end face 28, is
confined at inside of the light guide plate 25 and progresses at an
inner portion of the light guide plate 25 while repeating total
reflection by an upper face and the lower face of the light guide
plate 25. As shown by FIGS. 13a and 13b, light f3 impinging on the
diffusing pattern 29 at inside of the light guide plate 25, is
reflected by an interface of the diffusing pattern 29, emitted
toward a light emitting face 30 at a surface of the light guide
plate 25 and is emitted from the light emitting face 30 to outside
substantially orthogonally thereto by being incident on the light
emitting face 30 by an angle of incidence smaller than a critical
angle of total reflection. Or, as in light f3 shown in FIG. 13(a),
light f3 is emitted toward the light emitting face 30 at the
surface of the light guide plate 25 by being subjected to total
reflection twice continuously by the lower face of the light guide
plate 25, and is emitted from the light emitting face 30 in a
direction substantially orthogonal thereto.
The individual diffusing patterns 29 are uniform in a
circumferential direction centering on the light emitting portion
26 and therefore, as shown by FIG. 14(a), even when light incident
on inside of the light guide plate 25, impinges on the diffusing
pattern 29, in plane view, a progressing direction of light remains
unchanged and in the circumferential direction centering on the
light emitting portion 26, and there is not operation of diffusing
light. Therefore, uniformity of light in the circumferential
direction is determined only by a light amount distribution of the
light emitting portion 26 in the circumferential direction, or a
light amount distribution of light entering inside of the light
guide plate 25 from the light incident end face 28. As a result,
according to the plane light source apparatus 24, the directing
property of light in the circumferential direction is narrowed and
therefore, as shown by FIG. 14(b), a directivity of light emitted
from the light emitting face 30 of the light guide plate 25 is
extremely narrowed in a width direction of the light guide plate 25
and widened in a length direction thereof. In this way, when the
light guide plate 25 is viewed in a direction orthogonally thereto,
the directivity is narrowed in one direction (circumferential
direction of the diffusing pattern 29) and therefore more light can
be emitted vertically. A directing angle in the narrow direction is
preferably .+-.15.degree. through .+-.20.degree. (30.degree.
through 40.degree. as a whole) in view of easiness of seeing a
screen of the liquid crystal display apparatus 21.
Light emitted from the light emitting portion 26 and incident on
inside of the light guide plate 25, is widened radially in view
from the direction orthogonal to the light guide plate 25 and
therefore, an optical intensity is large at a vicinity of the light
emitting portion 26, and the more remote from the light emitting
portion 26, the smaller the optical intensity. Therefore, at an
area proximate to the light emitting portion 26, a pattern density
of the diffusing patterns 19 is reduced, at an area remote from the
light emitting portion 26, the pattern density of the diffusing
patterns 29 is increased to thereby make light easy to emit from
the light emitting face 30, thereby, a distribution of light
emitted from the light emitting face 30 of the light guide plate 25
is made uniform and a nonuniformity in brightness is made to be
difficult to bring about.
Further, at a lower face (face on a side opposed to a face opposed
to the light guide plate 25) of the prism sheet 27, as shown by
FIG. 10 and FIG. 11, there are formed recessed and projected
patterns 31 in a shape of a prism having a section in a triangular
shape. The recessed and projected pattern 31 is formed in a
circumferential shape centering on the light emitting portion 26
and the respective recessed and projected patterns 31 are formed in
a shape of concentric circles at constant pitches. A total of the
lower face of the prism sheet 27 is formed with a reflecting face
32 by a metal-deposited film vapor-deposited with Ag, Al, T, Au or
the like.
FIGS. 15(a) and 15(b) are a partially broken enlarged sectional
view showing an example of a resin-molded portion 33 of a prism
sheet 27 (prism sheet before forming the reflecting face 32) and a
rear view thereof. The prism sheet 27 is formed with the recessed
and projected patterns 31 having a depth of 8.8 .mu.m in the shape
of concentric circles and discretely at a period (pitch) of 30
.mu.m, and an angle of inclination of the recessed and projected
pattern 31 is 18.degree. and an angle made by contiguous inclined
faces thereof is 90.degree. (that is, the recessed and projected
pattern 31 is formed by a section in a shape of a rectangular
triangle). Further, the numerical values are pointed out as an
example and the invention is not limited to the numerical
values.
According to the prism sheet 27 discretely formed with the recessed
and projected patterns 31 as described above, it is preferable that
one or more of the recessed and projected patterns 31 correspond to
inside of one pixel of the liquid crystal display panel 22. By
corresponding one or more of the recessed and projected patterns 31
to one pixel of the liquid crystal display panel 22, a dispersion
of brightness among the respective pixels can be reduced. Further,
it is preferable that the period of the recessed and projected
pattern 31 is prevented from being a period of the diffusing
pattern 29 (or, a period smaller than the period of the diffusing
pattern 29) multiplied by an integer in order to prevent Moire
fringes from being brought about by interfering with the diffusing
patterns 29 of the light guideplate 25. Further, although in FIG.
15(b), there is shown the prism sheet 27 in which the recessed and
projected patterns 31 are divided in the circumferential direction,
the recessed and the projected patterns 31 may be continuous in the
circumferential direction.
Further, as shown by FIG. 16, light passing the diffusing pattern
29, or passing a flat portion at which the diffusing pattern 29 is
not present and emitted skewedly from the rear face of the light
guide plate 25, enters inside of the prism sheet 27 and is
reflected by the reflecting face 32. A shape of the lower face of
the prism sheet 27 (angle of the recessed and projected pattern 31
or the like) is designed such that light reflected by the
reflecting face 32 in this way, is reflected in a direction
orthogonal to the prism sheet 27. Therefore, light reflected by the
prism sheet 27, passes the light guide plate 25 and is emitted from
the light emitting face 30 of the light guide plate 25 in a
direction substantially orthogonal to the light emitting face 30.
As described above, light at inside of the light guide plate 25 is
provided with the narrow directivity in the circumferential
direction of the concentric circle centering on the light emitting
portion 26 and therefore, also light emitted skewedly from the
lower face of the light guide plate 25 is also provided with narrow
directivity in the circumferential direction, and light having the
narrow directivity is reflected by the reflecting face 32 of the
prism sheet 27 and therefore, loss of light from the lower face of
the light guide plate 25 can be minimized and light can be emitted
from the upper face of the light guide plate 25 similarly with
narrow directivity.
Further, according to the prism sheet 27, the recessed and
projected patterns 31 are formed in the shape of the concentric
circles centering on the light emitting portion 26 in conformity
with the diffusing patterns 29 of the light guide plate 25 and
therefore, an amount of light reflected by the prism sheet 27 is
made uniform in the face.
Light f3 reflected by the diffusing pattern 29 and emitted from the
upper face of the light guide plate 25, is emitted in a direction
substantially orthogonal to the light guide plate 25 (FIG. 13) and
its directing characteristic is shown by FIG. 17. Incidentally, in
FIG. 17, the abscissa designates an angle of light ray relative to
the direction orthogonal to the surface of the light guide plate 25
produced by viewing the light guide plate 25 from a side face
thereof and the ordinate designates an optical intensity
represented by an arbitrary unit (a.u.). As shown by FIG. 17, 55%
of a total amount of light emitted from the upper face of the light
guide plate 25, is included in a viewing angle 30.degree.
(.+-.15.degree.) regarded to be easy to see an image.
In contrast thereto, light transmitting the diffusing pattern 29,
or transmitting a flat portion which is not provided with the
diffusing pattern 29, and emitted from the rear face of the light
guide plate 25, is skewedly emitted to the rear face of the light
guide plate 25 as shown by FIG. 16, and its directing
characteristic is shown by FIG. 18. Incidentally, in FIG. 18, the
abscissa designates an angle .theta. of light ray relative to the
direction orthogonal to the rear face of the light guide plate 25
produced by viewing the light guide plate from a side face thereof,
and the ordinate designates an optical intensity represented by an
arbitrary unit (a.u.). As shown by FIG. 18, inclination of light
emitted from the lower face of the light guide plate 25, is about
68.degree. and a half value angle thereof is as narrow as
28.degree.. Therefore, when light emitted from the lower face of
the light guide plate 25 is incident on the prism sheet 27, light
having a spreading angle of 28.degree. is incident thereon by an
angle of incidence near to 90.degree., and as shown by FIG. 19,
light incident on the prism sheet 27, becomes light having a very
narrow directing angle of 9.degree. by being refracted by the
interface (upper face) of the prism sheet 27, reflected by the
reflecting face 32 while maintaining the directing angle and is
emitted from the upper face of the prism sheet 27 as light having a
narrow directing angle of 13.degree..
Although also light reflected by the diffusing pattern 29 of the
light guide plate 25, is emitted in the direction substantially
orthogonal to the light emitting face 30, a function (narrow
directivity) for aligning and emitting reflected light in the
direction substantially orthogonal to the light emitting face 30,
is more excellent in the case of the prism sheet 27 than in the
case of the diffusing pattern 29. Therefore, although as shown in
FIG. 20, the larger the pattern density of the diffusing pattern 29
provided at the rear face of the light guide plate 25, the more
increased is the light amount emitted from the light guide plate 30
(emitting light amount), a diffusing degree of light emitted from
the light emitting face 30 is increased and the directivity is
widened. Therefore, in order to narrow the directivity of light
emitted from the light guide plate 25, it is preferable that the
pattern density of the diffusing pattern 29 is equal to or smaller
than 40%. Meanwhile, in order to maintain the emitting light
amount, it is preferable that the pattern density is equal to or
larger than, for example, 10%. Therefore, it is preferable that the
diffusing patterns 29 of the light guide plate 25 are not
distributed so densely but is distributed discretely to thereby
produce flat portions among the diffusing patterns 29.
When the brightness of the plane light source apparatus 24
according to the invention placing the prism sheet 27 on the rear
face of the light guide plate 25, brightness of a plane light
source apparatus of Comparative Example 1 placing a while color
scatting sheet on the rear face of the light guide plate and the
brightness of the plane light source apparatus of Comparative
Example 2 placing a mirror face reflecting sheet on the rear face
of the light guide plate, are compared the following result is
obtained.
Prism sheet (The Invention) 175 a.u. While color scattering sheet
100 a.u. (Comparative Example 1) Mirror face reflecting sheet 104
a.u. (Comparative Example 2)
According to the result, the plane light source apparatus 24 of the
invention is about 1.75 times as bright as the normal plane light
source apparatus of Comparative Example 1 and Comparative Example
2.
Although according to the liquid crystal display apparatus 21 of
the invention, the diffusing plate 23 is not necessarily needed,
when the directivity of light emitted from the plane light source
apparatus 24 is excessively narrow, the directivity can be adjusted
by placing the diffusing plate 23 between the plane light source
apparatus 24 and the liquid crystal display panel 22.
Further, according to the liquid crystal display apparatus 21 of
the invention, the prism sheet 27 is placed at a position remotest
from the liquid crystal display panel 22 and therefore, even when
there is a defect having a size of about 0.2 mm on the prism sheet
27, the defect is difficult to see and yield in fabricating the
plane light source apparatus 24 is promoted.
Further, according to the liquid crystal display apparatus 21 of
the invention, there is produced a distance of about 1 mm between
the liquid crystal display panel 22 and the prism sheet 27 and
therefore, Moire fringes are difficult to be brought about between
the liquid crystal display panel 22 and the prism sheet 27.
Further, since the recessed and projected pattern 31 of the prism
sheet 27 is directed to a side opposed to the light guide plate 25,
there is a distance of about 0.1 mm between the diffusing pattern
29 of the light guide plate 25 and the recessed and projected
patterns 31 of the prism sheet 27 and also Moire fringes produced
by interference between the diffusing pattern 29 and the recessed
and projected patterns 31 is difficult to be brought about.
(Second Embodiment)
FIG. 21 is a disassembled perspective view showing a structure of a
liquid crystal display apparatus 41 according to other embodiment
of the invention. According to the liquid crystal display apparatus
41, a line light source 42 is used as a light emitting portion. The
plane light source apparatus 24 is arranged on a lower side of the
liquid crystal display panel 22 and the diffusing plate 23, the
line light source 42 comprises a fluorescent tube 43 and a
reflector 44 and the line light source 42 is made to be opposed to
the light incident end face 28 of the light guide plate 25
substantially over a total length thereof. Further, in
correspondence with use of the line light source 42, the lower face
of the light guide plate 25 is formed with the diffusing patterns
29 aligned in parallel with the line light source 42. Further, in
correspondence with the diffusing patterns 29 of the light guide
plate 25, also the recessed and projected patterns 31 of the prism
sheet 27 having a section in a triangular shape, are formed in
parallel with the line light source 42 at constant pitches. Even in
such the plane light source apparatus 24, light can be emitted in a
narrow viewing angle and the efficiency of utilizing light can be
increased. Further, similar to the case of the first embodiment,
Moire fringes can be prevented and a defect of the prism sheet 27
is made to be difficult to see.
Further, also according to the light guide plate 25, the pattern
density of the diffusing pattern 29 is small at a vicinity of the
line light source 42 and the remoter from the line light source 42,
the larger the pattern density becomes gradually. Therefore, also
in the plane light source apparatus 24, the brightness distribution
of the light emitting face 30 can be made to be uniform.
Also according to a plane light source apparatus shown in FIG. 22,
the line light source 42 is used as a light emitting portion, and
according to the line light source 42, a light emitting portion is
constituted by light of a point light source 46 converted into the
line light source 42. That is, the line light source 42 is
constituted by the point light source 46 of a light emitting diode
or the like and a light guide member 45 formed in a wedge-like
shape, and the point light source 46 is arranged to be opposed to
an end face of the light guide member 45. The light guide member 45
is molded by a transparent resin having a high diffractive index of
polycarbonate resin or methacrylic resin, a face (hereinafter,
referred to as front face) 47 opposed to the light incident end
face 28 of the light guide plate 25, is made to be in parallel with
the light incident end face 28, other face (hereinafter, referred
to as rear face) 48 is inclined to the light incident end face 28
and the remoter from the point light source 46, the narrower the
width of the light guide member 45 becomes gradually.
Further, light emitted from the point light source 46, enters
inside of the light guide member 45 from the end face of the light
guide member 45, progresses while repeating reflection by the two
faces 47 and 48 of the light guide member 45 and reflected by the
rear face 48 to thereby reduce an angle of incidence to the front
face 47. When the angle of incidence to the front face 47 becomes
smaller than the critical angle of total reflection, the light is
emitted from the front face 47 of the light guide member 45. In
this way, light is emitted from a total of the front face 47 of the
light guide member 45 substantially uniformly and light is made to
be incident on inside of the light guide plate 25 from the total of
the light incident end face 28 of the light guide plate 25.
(Third Embodiment)
FIG. 23 is an outline side view showing a liquid crystal display
apparatus 51 according to still other embodiment of the invention.
According to the embodiment, a surface (face on a side opposed to
the light guide plate 25) of the prism sheet 27 is formed with the
recessed and projected patterns 31, and the reflecting face 32 is
formed at a surface thereof by vapor deposition of metal. When the
reflecting face 32 is provided on the surface of the prism sheet
27, although the vertical brightness is reduced, since a viewing
angle is regarded to be easy to see when the prism sheet 27 is
30.degree. (.+-.15.degree.), about 70% of light of a total of
reflected light is brought into the viewing angle and a plane light
source apparatus 52 is sufficiently effective.
Further, when a direction of light reflected by the reflecting face
32 of the prism sheet 27 is not orthogonal to the prism sheet 27,
the prism sheet 27 may be inclined relative to the light guide
plate 25 such that light is reflected orthogonally to the light
guide plate 25 as in a liquid crystal display apparatus 53 shown in
FIG. 24.
According to the plane light source apparatus, an optical sheet for
reflecting light emitted from a rear face of a light guide plate in
a skewed direction and making the light incident on the rear face
of the light guide plate orthogonally thereto, is arranged
opposedly to the rear face of the light guide plate and therefore,
light leaked from the rear face of the light guide plate can be
reflected into a viewing angle, light which has been reflected to
outside of the viewing angle in the related art can also be
condensed into the viewing angle and the efficiency of utilizing
light can be increased.
Further, according to the plane light source apparatus, when the
plane light source apparatus is arranged on a rear side of an image
display panel, since the optical sheet is disposed at a position
remotest from the image display panel, even when there is a defect
on the optical sheet, the defect becomes difficult to see and Moire
fringes are difficult to be brought about between the image display
panel and the optical sheet.
* * * * *